Large bearing assemblies which have been developed for a variety of applications normally include grease fittings to permit the injection of grease or other lubricants into the bearing case. With such bearings, periodic attempts are made to inject additional lubricant into the bearing regardless of whether replenishment of the lubricant is actually required. This is both wasteful and time consuming, but has often proven to be the only method for insuring that a bearing remains properly lubricated short of complete bearing disassembly.
The problems involved in maintaining proper bearing lubricant supplies have been compounded by the development of permanently lubricated sealed bearings. Theoretically, these permanently lubricated sealed bearings are lubricated for life and consequently they are not provided with grease fittings to facilitate replenishment of the bearing lubricant. Practically however, such permanently lubricated sealed bearings also experience seal failure and lubricant leakage after long periods of use, particularly in harsh environments, and this condition is often difficult to detect unless the bearing is periodically disassembled for inspection. Periodic bearing disassembly and inspection is an extremely time consuming and expensive process for many bearing applications, as for example, in the case of railroad wheelset bearings. There is no method for effectively determining the grease content in the field of either a railroad wheelset bearing having a grease fitting or a permanently lubricated, sealed railroad wheelset bearing. To accomplish this, the wheelset must be removed from operation and disassembled for inspection.
To measure the lubricant content of a bearing in the field without disassembly, merely gaining access to the interior of the bearing case is not enough, for the grease or other lubricant will not be found in a single location to facilitate measurement. Instead, it will be splattered around within the closed interior of the bearing case.
Prior art devices have been developed to sense or measure the properties of bearing lubricant. U.S. Pat. Nos. 2,033,588 to Pigotti et al and 3,946,593 to Ruget show devices which are typical of those previously developed, but these units are not suitable for use in the field to measure the lubricant content of a sealed bearing.
The volume to be measured within a sealed bearing case is partially filled with liquid and the remainder is filled with air or gas. The volume within a closed container has been previously measured using a charge of air or gas, as illustrated by U.S. Patent Nos. 3,623,372 to Markey, 3,962,916 to Bouchy et al, 3,585,861 to Keng and 4,430,891 to Holm. These devices all disclose volume measurement techniques and devices which may operate effectively in controlled conditions, but which are not adapted for bearing field measurement conditions.